1. Field of Invention
This invention relates to a laser radar apparatus for use in a vehicle lateral guidance system. Such vehicle lateral guidance systems may be used, for example, in a steering controller for use in an automobile on an automated highway. Such automated highways are contemplated for providing automated control of speed, steering and braking of an automobile, replacing the human driver as the prime source of control inputs in an automobile in order to improve traffic flow and highway safety. In addition, the present invention may also be applied to a vehicle lateral guidance control system which may be used in conjunction with a manually controlled automobile to provide supplementary control signals relating to course correction in order to maintain proper lane tracking of the vehicle on a superhighway, counteracting effects due to crosswinds, uneven road surfaces, inattentive driver, etc.
The present invention may also be applied to vehicles other than automobiles such as robotic material handling transfer systems, automated sweepers, automated office mail delivery systems and the like. In addition the laser radar apparatus of the present invention has applications in vehicle navigational systems and surveying equipment.
2. Description of the Prior Art
Prior art vehicle guidance systems are well known for controlling robotic material handling systems (i.e., automated guided vehicles or AGVs) and to some extent, automobiles. These control systems typically utilize an inductive sensor mounted near the bottom of a vehicle to sense current passing through a thin wire mounted on or underneath a road surface. The inductive sensor senses the magnetic field produced by the wire and outputs a signal indicating how far the vehicle has drifted from the desired course. A control system converts these signals into control signals which are used to bring the vehicle back on course.
These inductive type systems have met with some success in industrial robotics applications, however, they have a few serious drawbacks. The inductive sensor is prone to picking up spurious signals generated by other electromagnetic sources such as electric motors, powerlines, etc. Further, the magnetic field produced by the roadway mounted cable can be "shielded" by metallic structures such as bridges, concrete reinforcement rods (so-called "rebar"), etc. In addition, such a system requires the installation and maintenance of a cable at or near the surface of the roadway. The installation of such a cable for any extended length of roadway would be prohibitively expensive. Further, maintenance of such a cable would require the installation of transformers, current regulators and power supplies at periodic intervals along the roadway. Finally, such a system is especially vulnerable to maintenance problems, such as a break in a cable (at an expansion joint, for example). Such a break could disable an entire section of roadway, which, if unexpected, could produce disastrous results.
In addition to the practical disadvantages of the inductive sensor type systems, there are also design limitations of such systems which severely limit their use. The inductive sensor is mounted on the vehicle and senses only that section of roadway immediately underneath the vehicle. As such, the minimum radius of any road curvature, or conversely the maximum speed of the vehicle is limited to a range within which the inductive sensor can respond. The inductive sensor type systems cannot, in effect, "look ahead" as a human driver does, evaluate the roadway ahead and act accordingly. Inductive sensor systems can only react to the information received from the roadway immediately beneath the vehicle.
In an attempt to overcome the disadvantages of these inductive sensor type systems, several devices have been proposed which use some type of optical sensor to detect a reflector, stripe, or some other sort of optical indicia applied at or near the road surface. An example of one such system is that of U.S. Pat. No. 4,703,240 issued Oct. 27, 1987 to Yoshimoto et al. Yoshimoto discloses an industrial vehicle with a light beam illuminating the area immediately beneath a vehicle. A series of photocells detect the reflected beam from a stripe applied to the road or floor surface. Deviation from the desired path is detected by these photocells, and correction signals are used to correct the vehicle course. This apparatus overcomes the disadvantages of the inductive type sensors in that the photocells are relatively immune to electromagnetic interference. In addition, a current carrying wire need not be placed at or near the surface of the roadway, but rather a easily maintained painted or applied stripe or series of markers is applied to the surface of or adjacent to the roadway. The Yoshimoto apparatus does share one disadvantage of the inductive devices in that the device only scans the section of roadway immediately beneath the vehicle. While such a system may be suitable for slow moving industrial vehicles where response time is not a problem, in an automotive environment, such a system has been found to be unsatisfactory, as high vehicle speeds necessitate that the steering controller be able to anticipate changes in the direction of vehicle travel, rather than merely respond to them.
One apparatus which attempts to overcome such difficulties is shown in U.S. Pat. No. 4,049,961 to Marcy. Marcy shows an automatic guidance system for an automobile wherein lasers are used to scan a limited region ahead of the automobile. Return signals produced by light reflected from roadway or roadside mounted retroreflectors are used to indicate the position of the vehicle relative to the roadway. Although the Marcy device is equipped to "look ahead" down the roadway, the output of the apparatus merely indicates the deviation of the vehicle position from an imaginary centerline of the roadway. This technique allows for some anticipation in directions in travel direction, however, the Marcy device cannot determine or calculate road curvature based upon the received reflected signals and thus anticipate and respond to changing road geometry.